1. Field of the Invention
This invention is directed to systems and methods for recovering hydrocarbons from the earth, and in one particular aspect to such recovery from diatomaceous and other hydrocarbon-bearing rock occurring at shallow depths and sometimes outcropping at the surface; to such systems and methods using recovery techniques involving the injection of substances and/or materials that improve the hydrocarbon recovery performance such as but not limited to steam injection; and, in one particular aspect, to such systems and methods including an artificial shield on a formation for reducing or eliminating the escape of injected materials and/or substances and/or pollutants to the surface and/or environment. In one aspect, the present invention is directed to a recessed wellhead system.
2. Description of Related Art
The prior art discloses knowledge of a variety of known liquid and solid hydrocarbon deposits that have not been exploited because of unfavorable economics or unavailable and/or inadequate technology. "Diatomaceous earth", "diatomaceous oil shale", and "diatomaceous rock" occurring at very shallow depths--collectively referred to herein as "diatomite"--is one type of this relatively unexploited unconventional petroleum resource. Diatomite is composed of the siliceous skeletal remains of single-celled marine plants or algae called "diatoms". There are known extensive deposits of hydrocarbon-bearing diatomite in California.
One such deposit is in the McKittrick Field in western Kern County, California situated in the northwestern end of a relatively narrow band of rich oil-bearing diatomite. The band is about 17 miles long and about one mile wide. It is estimated that the McKittrick area, one of the many areas of interest to which this invention applies, may contain over 800 million barrels of oil.
The majority of diatomite in the McKittrick Field occurs from the surface down to a depth of about 2000 feet, total vertical depth. Close to the surface, the accumulation tends to mainly consist of what is referred to as Opal A diatomite rock sometimes mixed with other sediment and rock material types. In addition, high concentrations of high viscosity and high density crude oil is also contained herein.
Opal A diatomite is known to have characteristics of very low permeability and very high oil concentrations when compared with conventional heavy oil-bearing sandstone rock successfully being developed in the area. However, the combination of very low rock permeability and high crude oil viscosity make it extremely difficult or virtually impossible to develop and produce this resource using conventional exploitation methods. This is confirmed by very limited and virtually non-existent resource development by operators owning rights to the resource accumulations.
Diatomite rock tends to change in characteristic form depending on the temperature at which the accumulation occurs and the amount of non-diatomite material that may be present. The higher the temperature and the more non-diatomite material present, the greater the tendency is for this change to occur. Since normal formation temperatures increase with depth according to the local geothermal gradient, observed diatomite form changes can be expected to behave accordingly. The resulting transformation is a more stable crystalline form often referred to as Opal CT. Opal CT normally begins to occur at depths ranging from 1000 to 2000 feet. The transformation is usually complete below the lower depth. One possible exception to this somewhat ordered tendency is the movement or displacement of the rock material caused by localized tectonic events such as faulting. These events can produce a re-ordering of the material and a perceived exception to the ordered behavior discussed above when compared with an undisturbed accumulation.
Opal A is amorphous non-crystalline diatomite composed of substantially unaltered and rubblized diatom fossils with a porosity of about 55% to about 70% and a permeability of tens of millidarcies. Opal CT diatomite is composed substantially of diagenetically-altered and broken diatom fossils with a porosity of about 35% to about 55% and a permeability of about one to about five millidarcys.
The unaltered nature of the Opal A diatomite fossils insures that not only are there hydrocarbon deposits in the voids between adjacent fossils, but also deposits in the voided fossil shell previously occupied by the soft parts of the living organism which comprises a large part of the diatom frustule volume.
Intact diatoms often settled in a hydrodynamically stable position on the ocean floors eons ago. This tends to result in a more regular, layered deposit, thus contributing further to the increased porosity of Opal A diatomite and its accompanying increased capacity for hydrocarbons.
Accordingly, a formation composed of Opal A diatomite tends to hold more hydrocarbons per unit bulk volume than a formation composed of Opal CT diatomite. Furthermore, Opal A and Opal CT diatomite forms contain significantly more hydrocarbons per unit bulk volume than a formation composed of predominantly sandstone rock material.
Generally speaking, the McKittrick diatomite typifies much of the oil bearing shallow diatomite occurring in California including but not limited to the following general characteristics: a cover of overburden that varies from nothing at various surface outcroppings to hundreds of feet of thickness; a vertical formation thickness ranging from a few feet to well over 1,200 feet; a formation base extending from the surface to depths of about 1000 to 2000 feet; an average porosity of about 65%; a permeability range of about 5 to 50 millidarcys; viscosity of the oil contained herein of about 3000 centipoise; and an oil concentration of as much as 2800 barrels per acre-foot. One area of interest in McKittrick is about 1680 acres--i.e., this oil accumulation contained in diatomite is relatively small in areal extent when compared with conventional heavy oil accumulations contained in sandstone rock. Yet, very limited and virtually non-existent resource development by operators owning rights to the resource accumulations has ever occurred.
The prior art discloses that a variety of hydrocarbon extraction methods have been considered for McKittrick and other shallow diatomite fields including, but not limited to, steam injection; hydraulic fracturing; and strip mining.
Hydraulic fracturing of the shallow McKittrick diatomite may produce ruptures to the surface, which may endanger personnel, cause oil spills, and vent hydrocarbon and other gases to the atmosphere.
Strip mining or open pit mining using solvent or retort extraction for the McKittrick diatomite may result in large volumes of gases dissolved in the crude being released to the atmosphere as new ore is exposed and the fluid pressure is released as the overburden is removed.
Regarding steam injection, the differences between conventional methods and what is disclosed in one particular embodiment of the present invention is presented by means of an example regarding the effects of the concentration of the resource and of the formation properties and the effect on pattern spacing.
Example I compares the oil-in-place in a representative 2.5 acre area in Kern River (a conventional field operation) and a 0.156 acre area in the McKittrick Field diatomite. With the units shown in the examples, oil-in-place is calculated to be the product shown below:
______________________________________ Oil-In-Place = 0.7758 .times. Porosity .times. Oil Saturation .times. Thickness .times. Drainage Area EXAMPLE 1 - OIL CONCENTRATION INVENTION CONVENTIONAL SPACING SPACING (e.g. McKITRICK (e.g. KERN RIVER DIATOMITE FORMATION) FORMATION) ______________________________________ Porosity, % 30 65 Initial Oil Saturation, % 55 Formation Thickness, feet 403 Drainage Area, acres 0.156 Oil-In-Place, barrels 174,555 Concentration, barrels/acre 69,822 1,117,152 ______________________________________
Barrels in Example 1 above are at surface conditions and assume a formation volume factor very close to 1.0 reservoir barrel per stock tank barrel.
Example 1 shows the same amount of oil-in-place in both the diatomite formation (with well spacing according to one aspect of the present invention) and with prior art well spacing in a typical unconsolidated sandstone formation, even though the pattern or drainage area for the diatomite is 1/16 (=2.5/0.156) the area of the typical formation. The oil concentration in barrels per acre in a given zone is 16 times larger for the diatomite than for a typical unconsolidated sandstone operation--1,117,152 barrels per acre versus 69,822 barrels per acre, respectively.
Prior art has not considered, recognized, suggested, or addressed this small well spacing in the diatomite. Yet, the low permeability and the low fluid pressure seen in the shallow diatomite indicate to the present inventors that small well spacing is needed to drain the available reserves over a reasonable period of time. Implementing this approach in a formation with the uniquely high oil concentration as seen in the diatomite supports the need to go to smaller spacing. This invention addresses this and other related considerations needed to make such a process feasible.